Solar top up and charge

Mike_J · Posted: Fri Apr 24, 2015 9:59 pm Post subject: Solar top up and charge

I have just picked up a 40W portable panel from Go Power
http://gpelectric.com/products/portable-solar-kits
I was thinking of using it to keep the batteries topped up when it is in my driveway and wondered where I should connect the charger to.
I have two batteries connected to a switch. What are the normal settings for battery 1 and 2 (starter and "house"?).
If I want to charge without having the switch on, do I have to connect directly to the batteries?
(electrical newbie!)

Mike_J · Posted: Fri Apr 24, 2015 10:14 pm Post subject:

This is what it looks like inside my lazarette:
https://picasaweb.google.com/107085608707498595852/24Apr2015?authuser=0&authkey=Gv1sRgCM7loeLc76T1Hg&feat=directlink
[/img]

journey on · Posted: Fri Apr 24, 2015 10:16 pm Post subject:

The charger you have came with a charge controller, according to their spec sheet. So you connect the solar panel to the charge controller and the charge controller to the battery. Make sure positive (red) goes to the + terminal and negative (black) goes to the - terminal, all the way from the panel through the controller and thence to the battery. Connect directly to the batteries and set the main switch to off.

Since you have 2 batteries to be charged simultaneously , they will have to be connected in parallel. However, to prevent one battery from discharging into another, at night when the solar panel isn't supplying power, you'll need to separate them using 2 diodes. Go to a electronic supply store and ask for 2 ea 40 watt diodes. Connect 1 ea between each battery and the charger, with the + side of the diode to the + terminal of the charger and the - terminal to the battery.

Or you could hook up the charger to one battery at a time and manually change between batteries every few days. And when you get tired of that, go get the diodes.

Boris

Mike_J · Posted: Fri Apr 24, 2015 11:38 pm Post subject:

thataway · Posted: Sat Apr 25, 2015 1:33 pm Post subject:

Boris, it looks as if the 40 watt solar panel does not have a controller (I may have missed it), but the 80 and 120 are specced with a controller.

Boris's idea with the diodes is a good one, but the diode will drop the voltage by about half a volt. If the panel is putting out 13.8 volts then the battery will be getting 13.3 volts--enough to float. The reality is that the 40 watt panel is putting out 18 volts open voltage (common for these solar panels), at 2.2 watts, (not much), and they should handle the diode adequately. I think this will work as long as there is no draw. There is another reason for the diodes, and that is back feed at night, which chad allow current to flow back from the battery to the solar panel at night, and actually discharge the battery. The diode will prevent this. (Some solar panels have this diode built in).

What you want to do is to check the voltage of the batteries, with full sun, in early morning and late afternoon, and after dark. Post those voltages here. If the battery is staying above 12.6 volts at night, and more during the day, then you will be fine.

Mike_J · Posted: Sat Apr 25, 2015 2:35 pm Post subject:

The 40W unit does have a controller which will allow things to trickle once the battery is charged.
I will experiment with charging the "house" battery, which I think is #2.

journey on · Posted: Sat Apr 25, 2015 3:06 pm Post subject:

Bob,

As Mike pointed out above, it does come with a controller. The problem is it appears to have a single output. And I assume that it is set for 13.6 VDC, as is the one I have. So, if you want to charge 2 batteries simultaneously, they've got to be in parallel and separated when the charger has no output to prevent one battery from discharging into another. I don't believe there's any disagreement on that, is there?

So there are 3 ways of separating the batteries: manual switches, or diodes, either on the solar panel output or the charger output. The manual switch is self-explanatory.

For putting the diodes on the solar panel output and then connecting the diodes directly to the batteries, note that the spec on those panels is 18 VDC operating voltage. So any voltage drop across the diode only helps overcharging the battery and those panels only work during the day so the overcharge is minimal.

The best way, and I've saved it for last, is to use a diode after the charger with a diode for each battery. Yes there's a voltage drop and I submit figures from a design manual I have: "When forward current flows through a solid state diode there is a small voltage drop across its terminals. A silicon diode has a typical voltage drop of 0.6 to 0.7 volts, while a Schottky diode has a voltage drop of 0.15 to 0.45 volts." Silicon is worst case and would reduce the max voltage to 13.0 volts still giving a battery a good charge. Or one can use Schottky diodes. These are commonly used in PV systems as "blocking diodes" between batteries.

And of course, this is Mike's call. My solar panels connect to the grid through an inverter out of which I stay. Except for the panel on the truck, which does a so-so job of keeping that battery charged.

Boris